glFrustum  multiply the current matrix by a perspective
matrix
void glFrustum(
GLdouble left,
GLdouble right,
GLdouble bottom,
GLdouble top,
GLdouble zNear,
GLdouble zFar );
Specify the coordinates for the left and right vertical
clipping planes. Specify the coordinates for the bottom
and top horizontal clipping planes. Specify the distances
to the near and far depth clipping planes. Both distances
must be positive.
glFrustum() describes a perspective matrix that produces a
perspective projection. The current matrix (see glMatrixMode())
is multiplied by this matrix and the result
replaces the current matrix, as if glMultMatrix() were
called with the following matrix as its argument: {2
zNear} / {right  left} 0
A 0
0 {2 zNear} / {top  bottom}
B 0
0 0
C D
0 0
1 0
A = {right + left} / {right 
left}
B = {top + bottom} / {top 
bottom}
C = {zFar + zNear} / {zFar 
zNear}
D = {2 zFar zNear} / {zFar 
zNear}
Typically, the matrix mode is GL_PROJECTION, and (left,
bottom, zNear) and (right, top, zNear) specify the
points on the near clipping plane that are mapped to the
lower left and upper right corners of the window, assuming
that the eye is located at (0, 0, 0). zFar specifies the
location of the far clipping plane. Both zNear and zFar
must be positive.
Use glPushMatrix() and glPopMatrix() to save and restore
the current matrix stack.
Depth buffer precision is affected by the values specified
for zNear and zFar. The greater the ratio of zFar to zNear
is, the less effective the depth buffer will be at distinguishing
between surfaces that are near each other. If r =
zFar / zNear
roughly log[2] (r) bits of depth buffer precision are
lost. Because r approaches infinity as zNear approaches 0,
zNear must never be set to 0.
GL_INVALID_VALUE is generated if zNear or zFar is not positive,
or if left = right, or bottom = top.
GL_INVALID_OPERATION is generated if glFrustum() is executed
between the execution of glBegin() and the corresponding
execution of glEnd().
glGet() with argument GL_MATRIX_MODE
glGet() with argument GL_MODELVIEW_MATRIX
glGet() with argument GL_PROJECTION_MATRIX
glGet() with argument GL_TEXTURE_MATRIX
glGet() with argument GL_COLOR_MATRIX
glOrtho(3), glMatrixMode(3), glMultMatrix(3), glPushMatrix(3), glViewport(3)
glFrustum(3G)
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